It is frequently useful and sometimes essential to be able to determine the amount of exposure to sunlight a particular site will receive during the course of the year. When selecting the location for a home, it is desirable to know whether particular areas will be sunny in the winter or if overhanging roof portions will shield windows from the southern exposure of the sun in the summer. If the structure is to use a solar energy collector, it is essential to know whether the solar energy collector will be exposed to sunlight on a year-round basis. The path the sun travels can frequently pass behind trees, hills, or structures placing portions of a specific site in shadows at some time of the year and for some portion of each day. Such problems occur not only in the winter when the sun is near the horizon and would be obscured by hills or structures, but also in the summer when the sun would be obscured by overhanging tall trees under which it would shine during the winter.
It is not only important to know whether anything will obscure sunlight, but it is also important to know how long sunlight will be obscured and at what time of day. Sun shines most intensely around solar noon and collectors that store energy should be exposed to solar radiation at that time. It is sometimes apparent that a known obstruction such as a tree or a steeple will cause a certain site to be in its shadow at some portion of each day, but it is not readily determinable at what time of day that site will be in the shadow or for how long the sun will be obscured from that site.
It is evident that selecting the proper site for a structure on a lot can be of great value to get maximum sun exposure if it is desired, to get minimum sun exposure if it is desired, or to obtain shade or exposure only at particular times of the day or particular times of the year. Such information is also useful in deciding where to plant trees and what type of trees to plant in order to obtain shade in the proper place and at the proper time of day.
In the past, devices have been available to predict sun exposure for a particular site. These devices have either been very expensive instruments or handheld devices which depend on siting with the eye along two points much as one would look along the sites of a riffle. The former devices are too expensive and the latter are not accurate.